Four San Diego business groups have united to sound the alarm bell that looming defense cuts could cripple the regional economy, where 142,000 people get Pentagon or Veterans Affairs paychecks and another 169,000 jobs are tied to military spending.

Layoff notices could be issued starting in October, as San Diego defense contractors brace for sequestration, a deficit-slashing move that would cut the U.S. military budget by an extra 10 percent starting in January.

Five pieces of the San Diego military picture are considered in jeopardy of significant cuts, in addition to as many as 30,000 defense-related jobs, according to an analysis by the San Diego Regional Economic Development Corporation, the San Diego Regional Chamber of Commerce, Connect and the San Diego Military Advisory Council.

The list:

• A third aircraft carrier now slated for the region.

• The Navy’s Space and Naval Warfare Systems Command, especially its systems center in Point Loma.

In recent weeks, the redevelopment of the Navy’s downtown Broadway complex has been added to the concerns column. That’s in response to Rep. Susan Davis, D-San Diego, who introduced legislation in May that opened the door for the naval regional headquarters to be moved to a military base, instead of being rebuilt on Broadway as the Navy has planned for years.

Local military officials are remaining silent on the group’s analysis. The Navy’s Southwest regional command declined to comment. A spokesman for the San Diego Marine Corps Recruit Depot referred queries to the Pentagon.

But the business coalition, which calls this effort “Operation San Diego,” said other regions are already moving aggressively to protect their assets. Sen. Lindsey Graham, R-S.C., recently told his constituents that the Marines might have to choose between its San Diego boot camp and one in his home state, for example.

“When you have states like Maryland and Virginia that are sending off letters together to Washington, with all four of their senators and both governors in alignment, and you look at California. … It’s just not happening. We realized we need to develop a plan,” said Mark Cafferty, president and chief executive of the San Diego Regional Economic Development Corporation.

“We knew the very first step is generating a stronger presence in D.C. We’re an awful long way from there.”

The EDC is negotiating to hire the same Washington lobbyist who represented the region during the 2005 round of U.S. military base closures.

The case to be made at the Pentagon focuses on San Diego’s importance to national defense strategy.

San Diego Bay is the principal home of the U.S. Pacific Fleet, with air and sea training ranges off the coast that are unsurpassed for close access. The Navy SEAL command is in Coronado, and all SEALs get their initial training on the Silver Strand.

About one-quarter of the Marine Corps is stationed in San Diego County, with an infantry division at Camp Pendleton, an air wing at Miramar Marine Corps Air Station and one of the Corps’ two boot camps off Pacific Highway.

With the U.S. military’s shift of focus to the Pacific, the overriding sentiment in military circles has been that San Diego is situated well to weather defense cuts — though it will likely share in the 20,000-person reduction already announced for the Marine Corps.

But the Operation San Diego group warns that sequestration will not be like past base closure scenarios, which involved input from Congress. There’s no indication yet if defense officials will cut every program by 10 percent, or use the budgetary knife with more discretion.

“Nobody knows the details on how this is going to be executed, except that it’s going to be painful,” said Larry Blumberg, executive director of the military advisory council.

Sequestration refers to $600 billion in defense cuts scheduled to start next year. It is the legacy of a deal forged last summer in response to partisan division over raising the national debt ceiling.

It calls for $1.2 trillion in automatic cuts over 10 years, split evenly between defense and non-defense spending.

In the first year, the Pentagon budget would reportedly take a $55 billion hit, about a 10 percent reduction.

The cuts were triggered by the failure late last year to reach a comprehensive deal on a deficit reduction plan.

Separately, the Defense Department is already scheduled to slash $492 billion over the coming decade under a budget agreement passed last summer.

The Pentagon has spent the past year warning that sequestration could mean possible cancellation of major weapons programs and the disruption of operations around the world.

National University’s Institute for Policy Research in San Diego estimates that if the 10 percent cutback was evenly distributed across the military, San Diego’s economy would have lost $3.5 billion in 2011.

“This would severely stagger San Diego’s still-struggling economy, and undoubtedly put the region back into recession,” said National University economist Kelly Cunningham.

While making San Diego’s case to the Defense Department, business officials also want to arouse interest in San Diego, where 25 percent of the region’s jobs touch military spending in some way, according to an analysis by the military advisory council.

They worry that the issue isn’t being discussed, despite its back-pocket impact locally.

“The chamber and EDC have many of our members who are either defense-related or just local businesses who actually thrive off of the military being here,” Cafferty said. “A hit to the military is a hit to them.”

Ruben Barrales, president and chief executive of the San Diego Regional Chamber of Commerce, said one place to start is with local elected officials.

“They are key,” he said. “We need them to be active in their support for the military in San Diego.”

Rep. Duncan Hunter, R-Alpine, and a member of the House Armed Services Committee, held a town-hall meeting in San Diego on the proposed defense cuts last week. He called out the media for not covering the issue more and other regional politicians for not speaking up about what he called an “economic bombshell.”

Davis, a Democrat also on the House Armed Services Committee, said through her spokesman that sequestration can be avoided if both parties come together.

Political analysts have said it’s unlikely a solution will be found before the November election, as neither side wants to appear to concede any ground.

“If sequestration does go forward, San Diego’s civic and business leaders have a history of coming together and making a strong case for why our region is a strategic asset for the Department of Defense,” Davis said in an emailed statement. “I expect nothing less this time around.”

Her press secretary, Aaron Hunter, also said that the Broadway Complex legislation allows the Navy to move its regional headquarters, but doesn’t force a move. U-T San Diego Publisher Douglas F. Manchester has a contract with the Navy to redevelop the Broadway parcel into a new Navy administrative building, hotels, offices and shops.

The project so far has been hindered by the poor financial climate and legal challenges.

A possible bright side of the sequestration debate is that prior rounds of military cutbacks have meant a concentration of defense assets in San Diego, such as the 1997 move of the SPAWAR headquarters from the Washington, D.C., area.

While the Operation San Diego coalition says that these cuts may not be like the past, Cafferty added, “If there is going to be consolidation, this is the place to be consolidated to.”